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Science Explains Why the Family Dog Eats Its Own Poop

Your poop-eating pup may have picked up this nasty habit from its wolf ancestors, study suggests.

It is a known, but rarely discussed fact that even adorable, cuddly, and family-minded dogs often eat their own poop. This has long befuddled and disgusted both dog owners and children, who wonder why Fido gets to chow down on his own excrement, when they can’t even have a cookie until they’ve washed their hands. Until now, it was also unclear why this dietary ouroboros kept going round and round. But scientists are on it and can now provide an evolutionary explanation for canine coprophagy.

Now, a new study reports that roughly 16 percent of pet dogs eat stool, and that products marketed to prevent coprophagy are almost never effective. As for why dogs do the doo, the authors put forth a fascinating theory: it’s a trait inherited from wolves, who devour poop in the den area as a way of consuming fecal-borne intestinal parasites before they become infectious.  

“It’s a really fascinating piece,” Elizabeth Krisch, an animal cognition researcher at Hunter College (who also just so happens to be my sister), tells Fatherly. “It’s normal to see coprophagy in a species, like rabbits, that require those extra nutrients, but in the case of dogs, this higher-order preventative and adapted intelligence is remarkable.”

Dogs mark their territory with excrement, and most canines know to stay away from poop. Nonetheless, “a puzzling, but common, behavior in some domestic dogs is a persistent tendency to consume their own feces or those of other adult dogs,” the authors write. And, while coprophagy doesn’t seem to harm the dogs, it certainly traumatizes the humans whom they lick.

“Dog owners are often very disturbed by the behavior,” the authors note. “In fact, as of this writing, there were 11 commercial products specifically marketed for dealing with the problem.”

So the authors conducted several large surveys, involving 3,000 dog owners. They asked owners how often their domestic dogs practiced coprophagy, whether any commercial products prevented it, and several other questions aimed at figuring out how this odd behavior developed. The results of their survey (while self-reported and thus limited) suggest that none of the commercial products on the market are effective, that 28 percent of dogs have eaten feces at least once, and that 16 percent of dogs have cheerily gulped down feces at least six times. And while no evidence linked coprophagy to the dog’s diet or age, one odd metric stood out. Even the most avid poop-eating pups barely sniffed at stool more than two days old.

Based on this finding, the authors raise the possibility that your gross pets are mirroring the behavior of wolves, who selflessly gobble up poop before fecal parasites can reach maturity and infect the pack—explaining dogs’ preference for poop no older than two days. “Leaving the feces alone would allow the ova to hatch into infective larvae that could be picked up on the hair of wolves and groomed off, thus transmitting the parasites,” the authors write. “If the feces are consumed while fresh, however, within about 2 days, the larvae will not yet have developed.”